Air filters are conventionally used to remove pollen, dust, and the like from the air, and nonwoven fabrics are often used as filter media for air filters. Among others, a melt-blowing process, which is one of the methods of producing nonwoven fabrics, is widely used to produce filter media of air filter products, battery separators and the like. A melt-blowing process is generally a process in which hot air is blown over a thermoplastic polymer extruded from a spinneret, whereby the polymer is formed into fine fibers, and the obtained fibers have self-fusing characteristics utilized to form the fibers into a fiber web. As compared to other production processes of nonwoven fabrics such as a spunbonding process or the like, a melt-blowing process has the advantages of requiring no complicated steps and easily producing fine fibers having a diameter of tens of micrometers to several micrometers or less.
The performance required to air filters includes high collecting efficiency that enables collection of a large amount of micro dust and low pressure loss that is due to low resistance to airflow passing through an air filter. To provide a filter medium having high collecting efficiency, a nonwoven fabric composed of single fibers having a small fineness is suitable, but single fibers formed to have a small fineness are likely to cause such a nonwoven fabric to easily collapse and increase the fiber density, resulting in causing an increased pressure loss.
In addition, to provide a filter medium exhibiting low pressure loss, a nonwoven fabric composed of single fibers having a thick fineness is suitable, but single fibers formed to have a thick fineness reduce the fiber surface area of such a nonwoven fabric, resulting in decreasing the collecting efficiency. Thus, having high collecting efficiency is incompatible with having low pressure loss.
To solve the above problem, an attempt has been made to satisfy both high collecting efficiency and low pressure loss by electretizing a nonwoven fabric and utilizing not only the physical action but also the electrostatic action.
For example, a method of producing electret fiber sheets has been proposed, in which the method includes: keeping a nonwoven fabric in contact with a ground electrode; and applying a high voltage to the nonwoven fabric with a non-contact voltage applying electrode while moving the ground electrode and the nonwoven fabric together, thereby continuously electretizing the fabric (see JP 61-289177 A). This proposition is based on a corona discharge method, in which a nonwoven fabric undergoes an injection of electrons, transfer of ions, orientation of dipoles, and the like in the fabric, whereby the nonwoven fabric is polarized and electrically charged. However, such a corona discharge method is a production method in which mainly only the surface of a fiber sheet is electrically charged and leaves it difficult to charge the inside with electric charge and, therefore, it is difficult to say that the method achieves sufficient electretization.
In view of this, as electretization methods of solving such a problem, methods in which electrically charging a fiber by subjecting it to water contact have been proposed. Specifically, proposed are: a method in which a fiber sheet is electretized by spraying water on the sheet using a water spout or water jet at a pressure sufficient for the water to permeate into the nonwoven fabric such that the fiber sheet has positive polarity charge and negative polarity charge evenly mixed (see U.S. Pat. No. 6,119,691 A); and a so-called hydrocharging method such as a method in which a fiber sheet is passed over a nozzle having a slit shape while the nozzle sucks water to thereby allow the water to permeate into the fiber sheet, such that the fiber sheet has positive polarity charge and negative polarity charge evenly mixed (see JP 2003-3367 A).
A fiber sheet obtained by the hydrocharging method can be electrically charged throughout the fiber sheet because even the fiber in the inside of the fiber sheet is subjected to water contact. Under the current circumstances, however, a fiber sheet cannot be densely charged with positive polarity charge and negative polarity charge even by using the hydrocharging method.
It could therefore be helpful to provide an electret fiber sheet formed by densely charging a fiber sheet with electric charge and having excellent dust collecting performance.